Tuesday, May 18, 2010

Another chink in the armor of the Standard Model?

Via Resonaances: The D0 collaboration has a new paper on the arXiv in which they present their observations of a like-sign muon charge asymmetryin B meson decays.

Neutral B mesons can decay into an antimuon, a mu neutrino and other stuff (B0 --> μ+νμ Xc) via the weak interaction \bar{b} --> \bar{c} W+, and neutral anti-B mesons can accordingly decay into an muon, a mu antineutrino and other stuff. However, neutral B mesons can oscillate into their antiparticles and back, so that if a B-Bbar pair is created in a collision, and one particle of the pair decays into a muon-neutrino pair while in its original state whereas the other decays into a muon-neutrino pair while turned into the antiparticle of its original state, both of them will decay into muons, or both into antimuons -- a like-sign muon decay.

If CP was an exact symmetry of nature, the rates for the oscillation and decays would be equal between B and anti-B mesons, but since it is not, CP violation leads to a difference in the rate at which the initial B-Bbar pair decays into positive and negative like-sign muon pairs -- a charge asymmetry. The Standard Model predicts a very small such charge asymmetry stemming from the complex phase in theCKM matrix.

What the D0 collaboration have done is to measure the charge asymmetry, carefully subtracting all (hopefully) sources of background, and obtained a result thatis about two orders of magnitude larger than the Standard Model prediction! Of course the experimental result has statistical and systematic errors, and thus the relevant measure of deviation from the Standard Model is only about 3σ ... still, this is another chink in the armor of the Standard Model.

What I find interesting is that all of the evidence of flavour physics beyond the Standard Model comes from particles containing a strange (rather than an up or down) quark besides a heavy flavour. The contribution to the charge asymmetry from B0d decays is well constrained by other experiments, so most of the D0 result would appear to be coming from the B0s system. I'm not a BSM phenomenologist, but I could imagine this to be relevant input for an understanding of possible BSM physics.

The Standard Model predictions rely on hadronic quantities such as decay constants, form factors and mixing parameters of the B meson, which must be determined nonperturbatively in lattice QCD. Better accuracy here could have real impact on the most stringent tests of the Standard Model that we have so far, and this is an area where significant progress is being made.

Monday, May 10, 2010

Bloggy stuff

This will be an unusaully bloggy post for this blog, consisting as it does of two unrelated remarks one of which is not terribly relevant to anything.

Firstly (and irrelevantly), I noticed that Google are now classifying the blogs on blogspot by some sort of content-matching algorithm -- if you click on "next blog" in the title bar, you are most likely going to see another physics blog, or at least a blog by some physicist; if that physicist happens to blog about cooking, the next blog after that might be another culinary blog, or if he happens to be based in Texas, it might be a blog about a trip to Texas, and so forth. That's a neat trick that makes the "next blog" feature actually at least somewhat useful.

Secondly, when people talk about how the Euro is loosing against the Dollar, which has to mean the end of the EU or even of civilisation as we know it, I wonder how short their memory spans are. Here's a little memory aid -- click on "10y" on the chart and behold, the Euro is 42.4% higher against the Dollar than it was in early 2000, when it was actually worth less than $1 ...

Thursday, May 06, 2010

ICHEP 2010 has a blog

As my readers will know, this blog is most active during the conference season, when I blog from the annual lattice conference and possibly also from other meetings. I believe that conference blogging is both a service to those members of the physics community who for whatever reasons cannot personally attend the conference, and also to the wider public, who can get an insight into what scientists do and talk about at their meetings. It is thus a great pleasure for me to be able to announce that ICHEP 2010 will have an official conference blog, where bloggers from the high energy particle physics community will post on the conference and on current topics in high energy physics in general.